[TowerTalk] SWR Problem Balun or lightning protection?

[Gene Fuller]

Also, as I have mentioned off line to Tony, some fixed capacitors will drift 
consirderably with temperature changes often caused by circulating current 
in tuned / matching circuits. Possibly a compensating capacitor in the 
balun - ?
Gene / W2LU

----- Original Message ----- 
From: "Tod - ID" <tod at k0to.us>
To: "Tony" <dxdx at optonline.net>
Cc: <towertalk at contesting.com>
Sent: Wednesday, December 21, 2011 10:50 AM
Subject: Re: [TowerTalk] SWR Problem Balun or lightning protection?


> This is an interesting thread which has gotten difficult to follow.
>
> Let me see if I understand it correctly.
>
> 1. When you transmit at high power either with a continuous carrier [ 
> RTTY] or with a high transmit duty cycle the VSWR you observe changes and 
> increases.
>
> I have some questions that I would seek to answer if it happened at my 
> station.
>
> 1. Is this something that I noticed only recently?
> 2. If the answer to (1) is yes, then how long ago do I think it was 
> operating correctly?
>
> 3. What has changed since I think it was operating correctly?
>  a. New mode or method of operating?
>  b. New equipment  - feed line, connectors, filters, relays, transformer 
> balun , current balun, lightning protection, antenna tuner, amplifier, 
> etc.
>  c. The outside temperature is different than it was when it last was 
> working correctly and the last time the temperature was the same as it is 
> now the system was working.
>
> If it was working and is now not working the cause should be associated 
> with a change and that change should be somewhere in the collection of 
> choices listed in (3) above.
>
> Two years ago I experienced a similar phenomenon when operating on 160 
> meters. I mesured the change in VSWR as a function of time when I 
> transmitted on air using a continuous RF carrier. I could observe the VSWR 
> rising as the length of transmission increased. I could also see that the 
> VSWR increased at a different rate when the temperature outside was 30 
> degrees F than when the temperature was 10 degrees F.
>
> The rate of VSWR rise changing with temperature led me to conclude that 
> the cause was something outside rather than inside the shack. The fact 
> that I had recently placed a trap in a top loading wire of the vertical 
> was another change. The trap had been fabricated to handle very high 
> voltages, but it simply was inadequate. The problem was dielectric heating 
> of the coil form. The heat would  dissipate more rapidly when it was cold. 
> I had to abandon that plan for my vertical.
>
> I know that ferrites will heat when they are used in some high current 
> situations. If the ferrite in your balun is being heated because you are 
> operating with a higher transmit duty cycle that might cause what you 
> observe. If there is a defect in the manufacture of the balun it might 
> have failed or might never have been capable of what you wish it too do. 
> In my case I tried an Unadilla trap but it too experienced dielectric 
> heating my particular application. [ my application was not the one it was 
> designed to handle]'
>
> Maybe something in the above will  help you resolve your problem .
>
>
> Tod, K0TO
>
>
>
> Sent from my iPad 2
>
>
>
>>
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